1. Field of the Invention
The present invention relates to a vacuum fluorescent display tube and a process of manufacturing the display tube. More particularly, the present invention is concerned with ribs or rib structures which support grid electrodes of such display tube and which surround fluorescent segments of the tube, and a process of fabricating such ribs or rib structures.
2. Discussion of the Related Art
A vacuum fluorescent tube is known, wherein a plurality of anodes disposed on a substrate are covered by respective fluorescent layers, which are selectively activated, namely, emit light or glow when they are struck by electrons generated or liberated from cathodes disposed above the anodes. The fluorescent layers when struck by the electrons from the cathodes emit light in the direction toward the cathodes, and an image provided by the activated fluorescent layers is viewed in the direction from the cathodes toward the fluorescent layers (anodes). This type of fluorescent display tube is capable of providing a clear image with a relatively low voltage to accelerate the electrons. Further, the use of different fluorescent materials for the fluorescent layers which emit lights of different wavelengths permits a color display of images. Owing to these advantages, the fluorescent display tube has been widely used as display devices on acoustic devices and on instrument panels of motor vehicles.
In the fluorescent display tube of the type indicated above, mesh grids are disposed between the anodes and cathodes, to control activation or glowing of the fluorescent layers or segments formed on the anodes at different positions on the display screen. Upon application of a positive voltage (accelerating voltage) to a given grid, the electrons generated from the cathodes are accelerated by the grid and strike the fluorescent layers right below that grid. However, the electrons reaching a grid to which a negative voltage (cutoff bias) is applied are blocked by that grid, and the fluorescent layers right below that grid will not glow.
The mesh grids are supported by suitable legs on the substrate such that each grid extends over an anode array consisting of a given number of anodes, with a suitable spacing between the anode array and the grid. The strength of the grid decreases with an increase in the area of the grid covering the anode array, and the grid tends to suffer from thermal deformation if the size of the grid is relatively large. The thermal deformation may lead to a problem such as reduced luminance of the fluorescent layers, and short-circuiting. Further, the grid having a mesh structure inevitably blocks some portion of the light emitted from the fluorescent layers, whereby the luminance of the fluorescent layers is lowered by the grid.
Another drawback which arises from the use of the mesh grids relates to the density of the anode arrays, namely, density of display elements per unit area of the display screen. Described more specifically, some of the electrons accelerated by the grid to which the accelerating voltage is applied may leak and strike some of the fluorescent layers right below the adjacent grid to which the negative cut-off bias voltage is applied. In this case, the fluorescent layers which are not required to glow may glow due to the leakage electrons. To avoid such erroneous activation of the fluorescent layers, the adjacent arrays of anodes (adjacent arrays of fluorescent layers) covered by the respective mesh grids should be spaced apart from each other by a relative large distance, for example, at least 2 mm. This spacing prevents the display elements (arrays of fluorescent layers) from being arranged with high density.
There has been proposed another type of fluorescent display tube wherein planar grids made of an electrically conductive material are formed on the substrate, so as to surround respective fluorescent layers. An example of this type of fluorescent display tube is disclosed in JP-A-3-52945. In the fluorescent display tube disclosed in this publication, anodes 122 are formed in a suitable pattern on a glass substrate 120, and fluorescent layers 123 are formed on the respective anodes 122, while planar grids 121a, 121b are disposed so as to surround the anodes 122, as shown in the cross sectional view of FIG. 10. This display tube, which does not use mesh grids, does not suffer from the problems due to the use of the mesh grids, namely, drawbacks due to thermal deformation of the mesh grids, and reduced luminance of the fluorescent layers due to blocking of light by the mesh grids.
However, the fluorescent display tube of FIG. 10 has some drawbacks. Namely, the anodes 122 should have a dummy peripheral portion located outside the periphery of the fluorescent layers 123, over a distance indicated at "O" in FIG. 10, so that the dummy portion of the anodes 122 assures intended activation of the fluorescent layers 123 over their entire areas including the peripheral portion. Further, there should be left a considerably large spacing P between the anodes 122 and the grid electrodes 121a, 121b, so as to prevent shorting therebetween. The distance "O" and spacing "P" necessarily result in a relatively large distance or spacing between the adjacent fluorescent layers 123, that is, a relatively large spacing between the adjacent display elements or segments. Thus, the fluorescent display tube of FIG. 10 suffers from the same problem as the known display tube using the mesh grids.
The conventional fluorescent display tube of FIG. 10 also has a drawback which arises from substantially co-planar relationship of the planar grids 121a, 121b with the fluorescent layers 123, which inevitably leads to reduced effects of acceleration and blockage of the electrons generated from the cathodes by application of respective accelerating and bias voltages (positive and negative voltages). This requires static driving of the grids 121. Even if dynamic driving or strobing of the grids 121 is possible, a relatively high bias voltage is required to block the electrons, requiring a high line voltage.
In view of the above drawback, there has been proposed a fluorescent display tube in which electrically insulating ribs are formed on the substrate so as to surround respective fluorescent layers, and grid electrodes are formed on the upper end faces of the ribs so that the grid electrodes are spaced from the upper surfaces of the fluorescent layers in the direction perpendicular to the plane of the substrate. An example of this type of display tube is disclosed in JP A-62-290050. According to this display tube, The function of the the grid electrodes to accelerate and block the electrons is comparatively improved even where the display elements are arranged with comparatively high density.
To form the ribs, grid electrodes and fluorescent layers in the display tube indicated above, electrically insulating and conductive layers which give the ribs and grid electrodes are first laminated on the substrate, and these insulating and conductive layers are subjected to a dry etching operation using an etching mask formed of a resist. Selected portions of the insulating and conductive layers which are not covered by the resist mask are removed by the dry etching, while the other portions covered by the mask are left, whereby the ribs and grid electrodes corresponding to the covered portions of the layers are formed. The ribs and the substrate cooperate to define recesses in which the fluorescent layers are subsequently formed. To form the fluorescent layers, the recesses are filled with a suitable filler (e.g., 1,3,5 trioxan, C.sub.3 H.sub.6 O.sub.3) which has a solid phase at a room temperature. The filler masses filling the recesses are coated with respective fluorescent layers which contain a photosensitive resin (UV-curable resin). The filler masses are then heated into a liquid phase so that the fluorescent layers are sunk through the liquid down to the bottoms of the recesses. Subsequently, the filler masses are further heated to a gaseous phase, so that only the fluorescent layers (on the anode layer on the substrate) surrounded by the ribs are left in the recesses. Then, the fluorescent layers are exposed to a ultraviolet radiation to cure the photosensitive resin, and are baked for bonding to the substrate (anode layer).
In the fabricating process of the display tube described above, the etching mask is placed on the electrically conductive layer for the grid electrodes, and the dry etching utilizing glass bead blast is effected through the mask, to remove the portions of the electrically conductive and insulating layers which are not covered by the mask. Thus, the recesses are formed in the laminated conductive and insulating layers. However, the dry etching process utilizing glass bead blast does not enable the aspect ratio (depth/width) of the recesses to be larger than 2. This means that it is difficult to locate the grid electrodes at a level sufficiently high with respect to the fluorescent layers formed on the anode layer on the substrate. Thus, the spacing between the grid electrodes and the fluorescent layers is not sufficient to enable the grid electrodes to accelerate and block the electrons with high stability. Further, the glass bead blast tends to damage the anode layer at a final stage of etching, leading to deterioration of the anodes.